.box{
    position: relative;
}
.smallbox{
    border:1px solid #ccc;
    width: 200px;
    height: 200px;
}
.smallbox img{
    width: 100%;
    height: 100%;
}
.mask{
    width: 100px;
    height: 100px;
    background: rgba(255, 255, 255, 0.5);
    position: absolute;
    left: 0px;
    top: 0px;
}
.bigbox{
    position: absolute;
    top: 0px;
    left: 210px;
    display: none;
    border: 1px solid #ccc;
    width: 400px;
    height: 400px;
    overflow: hidden;
}
.bigbox img{
    position: relative;
    left: -400px;
    height: -400px;
}